This invention relates to an apparatus and method for removing volatile and semi-volatile contaminants from solid materials and, more particularly, to an apparatus and method for thermal desorption of organics and volatile metals from soils to separate the contaminants from the soil.
The contamination of soils, sludges, ashes, and other solids by organics and heavy metals is a significant environmental problem. Due to the large volumes involved and expensive disposal costs for these solids, there is a need to reduce the volume of waste requiring disposal.
The contaminated soil may be treated by destruction of the contaminants, such as by incineration or by pyrolysis, or by separation of the contaminants from the soil, such as by thermal desorption. During incineration, the contaminants are heated under oxygen concentrations and residence time to a temperature effective to decompose the contaminants. During pyrolysis, the contaminants are heated in the absence of oxygen for a predetermined residence time to a temperature effective to decompose the contaminants. During thermal desorption, the contaminants are heated under oxygen concentrations and residence time to a temperature effective to avoid decomposition of the contaminants, thereby enabling the separation of the volatilized contaminants from the soil.
An example of thermal desorption of soil is disclosed in U.S. Pat. No. 5,230,167 (Lahoda et al.), entitled "Removal of Organics and Volatile Metals From Soils Using Thermal Desorption" and assigned to the assignee of the present invention. Contaminated material, positioned on a belt conveyor, is vaporized by heaters such as infrared or microwave heaters providing high heat transfer rates (5 to 10 cal./sec./cm.sup.2), in a thermal desorption chamber. This is able to volatilize contaminants difficult to vaporize, and produce a substantially decontaminated processed material.
In another invention involving thermal desorption, U.S. Pat. No. 5,188,041 (Noland et al.) uses heated oil, water, or eutectic salts passing through rotating hollow shafts and screw conveyor type flights to transport and heat contaminated soil in a vapor stripper under sealed conditions. Such a process would have a low heat transfer rates and be effective primarily for highly volatile contaminants. Moisture and contaminants in the material are stripped and carried cocurrently with the material by separate, heated, non-oxidative transport gas fed into the stripper from an outside heater. The contaminants are then burned in a burner or thermal destructor before being wet scrubbed or passed through activated carbon and vented. The hopper transporting the highly volatile contaminated material is itself sealed. Rotating means, rather than a flat type conveyor, move the material through the vapor stripper, and the separate, non-oxidative, transport gas is introduced downstream from the contaminated material introduction, to prevent undue drying of the material and formation of a dry crust on its surface. Condensation inside the vapor stripping conveyor is specifically avoided, and the non-oxidative transport gas, externally introduced, is blown directly onto the top of the contaminated material, which may enhance the escape of volatiles.
Another example of thermal treatment of soil is disclosed in U.S. Pat. No. 4,738,206 (Noland). Contaminated soil is conveyed through a chamber by a screw conveyor with internally heated flights and vapor stripped at a temperature below the boiling temperatures of the contaminants. This approach, however, will result in the production of very large volumes of gas, which then must be treated for contaminant removal. U.S. Pat. No. 3,432,397 (Berg) discloses a method for heating a mass of solid particles to produce carbon residue and distillation products. Here, gas flows through the bed of solids to obtain the required heat transfer. U.S. Pat. No. 4,756,092 (Anderson) discloses a method for drying sludge by air injection during heating by a gas burning apparatus.
The EPA Applications Analysis Report, Shirco Infrared Incineration System, June 1989, pages 39-42, discloses utilizing infrared heating elements to heat material positioned on a conveyor belt, which results in desorption of the contaminants from the material within a primary chamber, followed by incineration of the desorbed contaminants in the primary chamber. If combustion of the desorbed contaminants is not complete within the primary chamber, then the residuals from the desorbed contaminants are incinerated in a secondary chamber, which may produce toxic combustion products.
Thermal desorption of soil is also disclosed in U.S. Pat. Nos. 4,977,839 (Fochtman et al.) and 4,864,942 (Fochtman et al.). Contaminated materials, which are placed in a rotary kiln, are subjected to a temperature effective to volatilize the contaminants, but below the incineration temperature, for a period of time sufficient to effect the desired degree of separation of contaminants. However, incineration is due to a combination of the presence of an oxidizing substance and the residence time of the material at a particular time, and is not due to the temperature at which the process occurs.
U.S. Pat. No. 4,782,625 (Gerken et al.) discloses volatilizing organic compounds in a rotating cylinder having plural flights for moving soil through the cylinder. The material drying means, filtering means, scrubbing means, and bed of activated carbon are individually mounted on the bed of trailers to facilitate transportation of the equipment. Another description of processes to treat contaminated soil using a rotary kiln can be found in Contaminated Land Reclamation and Treatment, Michael A. Smith ed., Plenum Press, pages 37-90, "On-Site Processing of Contaminated Soil" by W. H. Rulkens, 1985. Incineration, treatment with catalysts, and low temperature vaporization of contaminants are disclosed. In low temperature treatment of off-gas, Rulkens discloses the use of steam as a carrier gas, cyclone filtering, condensing, gas scrubbing, separation of gas from the resulting contaminated liquid, and filtering the gas through activated carbon before discharging the gas.
However, utilization of any rotating means, such as the rotary kiln, the rotating cylinder, or screw conveyor type flights has several disadvantages. Due to the tumbling of the material within the drum, which may cause portions of the material to progress through the drum at different rates, the residence time of the material within the rotating drum to effect the desired degree of separation is variable.
Another disadvantage to the utilization of the rotary kiln and the screw conveyor is the low heat transfer rates (about 0.2 to 1 cal./sec/cm.sup.2) through the shell of the kiln or through the hollow flights of the screw, which limits the throughput of these processes. Also, the soil or debris final temperature is limited by the materials of construction of the kiln and screw conveyor because the flame temperature required to obtain the desired heat transfer through the material to volatilize the contaminants may cause damage to the kiln and screw conveyor, such as material fatigue and melting of the rotary kiln and screw conveyor. Utilization of a lower flow temperature requires much larger pieces of equipment or a reduction of capacity. Use of lower soil temperatures to volatilize the contaminants requires a longer residence time of the material within the thermal desorption unit, and may result in incomplete contaminant removal.
Another disadvantage to the utilization of a rotating drum is the production of dust within the drum due to the tumbling of the material, which makes it difficult to treat the off-gas, produces a large amount of contaminated waste and can lead to operational problems, such as pipe blockages.
Therefore, what is needed is an improved apparatus and method that separates difficultly vaporized contaminates from the contaminated material, that has an increased throughput, that does not produce additional toxic materials, and that efficiently cools the processed material without recontaminating such material.